Antique sextant technology was perfected after the invention of the octant in the 1730s. An octant has an arc of 1/8 of a circle, or 45°; a sextant's arc is 1/6 of a circle (60°) and proved the most popular of the various instruments that would be devised (there were octants, quintants, and quadrants, each with a different arc).
The purpose of the antique sextant was to measure the apparent distance of an object (often a celestial object- the sun, a star, etc) above the horizon. This was done by first pointing the instrument in the general direction of the horizon, then sliding a moveable piece down until the object in question was visible in the bifurcated-view scope. Then, once the object was visible, the sextant was tilted back and forth to make sure it was in the center. Then it was just a matter of reading the number where the moveable piece had been stopped along the arc.
By taking a measurement of the sun at noon, a vessel's latitudinal position could be ascertained. By comparing two objects of known location, the boat's position could be triangulated from the readings of the sextant. Basic and practical, the sextant is still used as a backup tool for navigation on boats worldwide.
An antique sextant will generally be of the half-horizon or "traditional" type, which divides the view in the telescope in half using two mirrors, the horizon showing on one side and the object being spotted on the other - similar to a split-screen TV broadcast. Whole-horizon sextants show the entire horizon but are usually seen as less versatile than the half-horizon variety.
Collectors buy an antique sextant generally for its value as a rare vintage piece rather than a tool for navigation. This is due to several factors, not the least of which is that a sextant is a very delicate instrument, and any slight bend in the arc will cause it to be unreliable at best. (Modern mariners guard their sextants like possessive hawks, keeping them from harm and children, and take great care when removing their sextants from their cases for use.) Antique sextants' appeal is due to the classic workmanship and the specific history of each piece.
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